This invention relates generally to microfabrication technology and, more specifically, to coaxial transmission line microstructures and to methods of forming such microstructures using a sequential build process. The invention has particular applicability to devices for transmitting electromagnetic energy and other electronic signals.
The formation of three-dimensional microstructures by sequential build processes has been described, for example, in U.S. Pat. No. 7,012,489, to Sherrer et al (the '489 patent). The '489 patent discloses a coaxial transmission line microstructure formed by a sequential build process. The microstructure is formed on a substrate and includes an outer conductor, a center conductor and one or more dielectric support members which support the center conductor. The volume between the inner and outer conductors is gaseous or vacuous, formed by removal of a sacrificial material from the structure which previously filled such volume.
For communication between the coaxial transmission line microstructures and the outside world, a connection between the coaxial transmission line and an external element is needed. The transmission line may, for example, be connected to a radio frequency (RF) or direct current (DC) cable, which in turn may be connected to another RF or DC cable, an RF module, an RF or DC source, a sub-system, a system and the like. In embodiments, the term “RF” should be understood to mean any frequency being propagated, specifically including microwave and millimeter wave frequencies.
Structures and methods for such external connection are not currently known in the art. In this regard, the process of connecting an external element to a coaxial transmission line microstructure is fraught with problems. Generally, the microstructures and standard connector terminations differ significantly in size. For example, the inner diameter of the outer conductor and outer diameter of the center conductor of a coaxial transmission line microstructure are typically on the order of 100 to 1000 microns and 25 to 400 microns, respectively. In contrast, the inner diameter of the outer conductor of a standard connector such as a 3.5 mm, 2.4 mm, 1 mm, GPPO (Corning Inc.), Subminature A (SMA), K (Anritsu Co.), or W (Anritsu Co.) connector is generally on the order of 1 mm or more, with the outer diameter of the inner conductor being determined by the impedance of the connector. Typically, microfabricated coaxial transmission lines have dimensions that may be from two to more than ten times smaller than the smallest of these standard connectors. Given the rather large difference in size between the microstructure and connector, a simple joining of the two structures is not possible. Such a junction typically produces attenuation, radiation, and reflection of the propagating waves to a degree that is not acceptable for most applications. A microfabricated transition structure allowing mechanical joining of the two structures while preserving the desired transmission properties, such as low insertion loss and low return reflections over the operating frequencies would thus be desired.
Adding to the difficulty of microstructure connectivity is the relatively delicate nature of the microstructures when considering the forces typically exerted on such connectors. The microstructures are formed from a number of relatively thin layers, with the center conductor being suspended in a gaseous or vacuous core volume within the outer conductor. Although periodic dielectric members are provided in the described microstructures to support the center conductor along its length, the microstructures are still susceptible to breakage and failure caused by excessive mechanical stresses. Such stresses would be expected to result from external forces applied to the microstructures during connection with large external components such as repeated mating with standard connectors.
Still further, when transitioning between the coaxial transmission line and another element through which an electric and/or electromagnetic signal is communicated, signal loss due to attenuation and return reflection can be problematic. In addition to loss of signal, return reflection can cause failure of circuits and/or failure of circuits to perform properly. Accordingly, a transition structure which allows for coupling of coaxial transmission line microstructures to external elements which preserves the desired transmission properties over the frequencies of operation without significant signal degradation due, for example, to attenuation and reflections is desired.
There is thus a need in the art for improved coaxial transmission line microstructures and for their methods of formation which would address one or more problems associated with the state of the art.